Aqueous chlorous acid solution for use as disinfectant

ABSTRACT

A process for producing aqueous chlorous acid solution in which chlorous acid, which is safe for the human body, is easy to handle, and less generates chlorine dioxide, is yielded and used as a disinfectant for a pretreatment in food processing. To an aqueous sodium chlorate solution is added sulfuric acid or an aqueous solution thereof in such an amount and concentration that the pH of the aqueous solution can be kept at 2.3-3.4 to thereby react them and generate chloric acid. Subsequently, hydrogen peroxide is added to the chloric acid in an amount which is equal to or larger than the amount necessary for a reduction reaction to thereby yield chlorous acid. Any one of inorganic acids, inorganic acid salts, organic acids, and organic acid salts, or two or more thereof, or a combination or these is added to the aqueous solution containing chlorous acid yielded, whereby the chlorous and acid can be present for long and the pH of the aqueous solution is regulated to regulated to 3.2-7.0. Thus, high bactericidal power is imparted thereto.

TECHNICAL FIELD

The present invention relates to a process for producing an aqueouschlorous acid solution used for disinfection/sterilization of food forpretreatment in food processing operations and related facilities.

BACKGROUND ART

Conventionally, chlorine oxides (e.g., chlorine, hypochlorous acid,chlorous acid, and chlorine dioxide) are primarily used for disinfectionor sterilization of food for pretreatment in food processing operations,such as fresh perishable food including vegetables and fruits, and thefacilities related to processing and production of processed food, suchas containers, preparation/cooking machinery, and plant equipment. Ofthese, chlorine and hypochlorous acid, when reacted with organiccompounds, are known to produce trihalomethanes, which are carcinogeniccompounds. This, along with recent health-consciousness trend, hasfocused attention on acidified sodium chlorite (ASC) solution, which wasdeveloped in the United States of America and which possesses a highbactericidal effect and is less associated with trihalomethane-relatedadverse effects.

Reference 1: U.S. Pat. No. 6,524,624

To produce the above-mentioned ASC solution, an aqueous chlorous acidsolution is mixed with an acid known as “generally recognized as safe”(GRAS) and adjusted to pH 2.3 to 3.2.

However, the main active component of the above-mentioned ASC solution,chlorous acid, decomposes a short time after preparation due to its lowstability, thereby reducing its bactericidal potential. Theabove-mentioned ASC solution, therefore, needs to be preparedimmediately before use.

This preparation procedure is not only inconvenient but also associatedwith the disadvantages resulting from production of chlorine dioxidegas, which is highly likely to have toxic effects on individuals whoinhale it and corrosive effects on food-processing and cooking machineryand other related equipment.

DISCLOSURE OF INVENTION Problem to be Solved by Invention

The present invention was made taking into account the abovedisadvantages. The purpose of the present invention is to provide aneasy-to-handle, long-acting, stable aqueous chlorous acid solution.Another purpose of the present invention is to provide a disinfectantfor use in pretreatment of food-processing operations that releases areduced amount of chlorine dioxide, is safe to human health, andpossesses a high bactericidal activity.

Means for Solving Problems

In order to solve the aforementioned problems, a first feature of theprocess of the present invention is to employ a process for producing anaqueous chlorous acid solution for use as disinfectant, comprising:reacting an aqueous sodium chlorate solution with a volume andconcentration of sulfuric acid or aqueous solution thereof appropriatefor maintaining pH of said aqueous solution at 2.3 to 3.4, therebygenerating chloric acid, and subsequently adding thereto at least anamount of hydrogen peroxide required for reducing said chloric acid toproduce chlorous acid.

A second feature of the process of the present invention is to employ aprocess for producing an aqueous chlorous acid solution for use asdisinfectant, comprising: reacting an aqueous sodium chlorate solutionwith a volume and concentration of sulfuric acid or aqueous solutionthereof appropriate for maintaining pH of said aqueous solution at 2.3to 3.4, thereby generating chloric acid, subsequently adding thereto atleast an amount of hydrogen peroxide required for reducing said chloricacid to produce chlorous acid, and adding to the resulting aqueoussolution at least one compound selected from the group consisting ofinorganic acids and salts or a combination thereof, to adjust its pH inthe range of 3.2 to 7.0.

A third feature of the process of the present invention is to employ aprocess for producing an aqueous chlorous acid solution for use asdisinfectant, comprising: reacting an aqueous sodium chlorate solutionwith a volume and concentration of sulfuric acid or aqueous solutionthereof appropriate for maintaining pH of said aqueous solution at 2.3to 3.4, thereby generating chloric acid, subsequently adding thereto atleast an amount of hydrogen peroxide required for reducing said chloricacid to produce chlorous acid, and adding to the resulting aqueoussolution at least one compound selected from the group consisting ofinorganic and organic acids and salts or a combination thereof, toadjust its pH in the range of 3.2 to 7.0.

A forth feature of the process of the present invention is to employ aprocess for producing an aqueous chlorous acid solution for use asdisinfectant, comprising: reacting an aqueous sodium chlorate solutionwith a volume and concentration of sulfuric acid or aqueous solutionthereof appropriate for maintaining pH of said aqueous solution at 2.3to 3.4, thereby generating chloric acid, subsequently adding thereto atleast an amount of hydrogen peroxide required for reducing said chloricacid to produce chlorous acid, adding to the resulting aqueous solutionat least one compound selected from the group consisting of inorganicacids and salts or a combination thereof, and further adding at leastone compound selected from the group consisting of inorganic and organicacids and salts or a combination thereof, to adjust the pH in the rangeof 3.2 to 7.0.

A fifth feature of the process of the present invention is to employ theprocess for producing an aqueous chlorous acid solution for use asdisinfectant, wherein said inorganic acid according to any of second toforth features of the present process, include carbonic acid, phosphoricacid, boric acid, or sulfuric acid.

A sixth feature of the process of the present invention is to employ theprocess for producing an aqueous chlorous acid solution for use asdisinfectant, wherein said inorganic salts according to any of second tofifth features of the present process include carbonates, hydroxides,phosphates, or borates.

A seventh feature of the process of the present invention is to employthe process for the process for producing an aqueous chlorous acidsolution for use as disinfectant, wherein said carbonates according tothe sixth feature of the present process include sodium carbonate,potassium carbonate, sodium bicarbonate, or potassium bicarbonate.

A eighth feature of the process of the present invention is to employthe process for the process for producing an aqueous chlorous acidsolution for use as disinfectant, wherein said hydroxides according tothe sixth or the seventh feature include sodium hydroxide or potassiumhydroxide.

A ninth feature of the process of the present invention is to employ theprocess for producing an aqueous chlorous acid solution for use asdisinfectant, wherein said phosphates according to any one of sixth toeight features include disodium hydrogenphosphate, sodiumdihydrogenphosphate, trisodium phosphate, tripotassium phosphate,dipotassium hydrogenphosphate, or potassium dihydrogenphosphate.

A tenth feature of the process of the present invention is to employ theprocess for producing an aqueous chlorous acid solution for use asdisinfectant, wherein said borates according to any one of sixth toninth features include sodium borate or potassium borate.

A eleventh feature of the process of the present invention is to employthe process for producing an aqueous chlorous acid solution for use asdisinfectant, wherein said organic acids according to any one of thirdto tenth features include succinic acid, citric acid, malic acid, aceticacid, or lactic acid.

A twelfth feature of the process of the present invention is to employthe process for producing an aqueous chlorous acid solution for use asdisinfectant, wherein said organic salts according to any one of thirdto eleventh features include sodium succinate, potassium succinate,sodium citrate, potassium citrate, sodium malate, potassium malate,sodium acetate, potassium acetate, sodium lactate, potassium lactate, orcalcium lactate

Advantageous Effect of the Invention

According to the present invention, there can be provided an aqueouschlorous acid solution which is highly disinfectant and stable, so thatit will not need to be prepared immediately before being used and it ismade possible to be preserved for future use. In addition, it preventsgeneration of chlorine dioxide so as to be harmless to human body andcan be used without anxiety.

Moreover, the aqueous chlorous acid solution produced in accordance withthe present invention can maintain prolonged stability and can bemarketed as disinfectant commodities.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the results of spectrophotometric measurement of sample Aconducted on the day of preparation.

FIG. 2 shows the results of spectrophotometric measurement of sample Aconducted on day 10 of preparation.

FIG. 3 shows the results of spectrophotometric measurement of sample Aconducted on day 20 of preparation.

FIG. 4 shows the results of spectrophotometric measurement of sample Aconducted on day 30 of preparation.

FIG. 5 shows the results of spectrophotometric measurement of sample Bconducted on the day of preparation.

FIG. 6 shows the results of spectrophotometric measurement of sample Bconducted on day 10 of preparation.

FIG. 7 shows the results of spectrophotometric measurement of sample Bconducted on day 20 of preparation.

FIG. 8 shows the results of spectrophotometric measurement of sample Bconducted on day 30 of preparation.

FIG. 9 shows the results of spectrophotometric measurement of sample Cconducted on the day of preparation.

FIG. 10 shows the results of spectrophotometric measurement of sample Cconducted on 1 hour of preparation.

FIG. 11 shows the results of spectrophotometric measurement of sample Cconducted on day 1 of preparation.

FIG. 12 shows the results of spectrophotometric measurement of sample Cconducted on day 5 of preparation.

FIG. 13 shows the results of spectrophotometric measurement of sample Dconducted on the day of preparation.

FIG. 14 shows the results of spectrophotometric measurement of sample Dconducted on day 10 of preparation.

FIG. 15 shows the results of spectrophotometric measurement of sample Dconducted on day 20 of preparation.

FIG. 16 shows the results of spectrophotometric measurement of sample Aconducted on day 30 of preparation.

FIG. 17 compares the time-course changes in pH values of the aqueouschlorous acid solutions of Examples 2, 3, and 4 and conventional ASCsolution.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the preferred embodiments of the present invention will bedescribed with reference to accompanying figures and tables.

Example 1

Example 1 of the present invention provides a process for producing anaqueous chlorous acid (HClO₂) solution for use as disinfectant.According to this process, sulfuric acid (H₂SO₄) or aqueous solutionthereof is added to an aqueous sodium chlorate (NaClO₃) solution tocreate acidic conditions, thereby generating chloric acid (HClO₃), andthe resulting chloric acid undergoes a reduction reaction with an excessamount of hydrogen peroxide to produce chlorous acid (HClO₂). Theessential chemical reactions of this production process are presented bythe following equations A and B:[CHEMICAL FORMULA 1]2NaClO₃+H₂SO₄→2HClO₃+Na₂SO₄↓  (Equation A)HClO₃+H₂O₂→HClO₂+H₂O+O₂↑  (Equation B)

Equation A indicates that chloric acid is generated by addition of anamount and concentration of sulfuric acid (H₂SO₄) or aqueous solutionthereof appropriate for maintaining the pH of the aqueous sodiumchlorate (NaClO₃) solution at 2.3 to 3.4, while sodium ions areeliminated concurrently.

Then, equation B shows that chloric acid (HClO₃) undergoes a reductionreaction with hydrogen peroxide (H₂O₂) to produce chlorous acid (HClO₂).This reaction requires the addition of at least an amount of hydrogenperoxide (or aqueous solution thereof) stoichiometrically required forthe reduction reaction. Otherwise, the reaction will yield only chlorinedioxide.[CHEMICAL FORMULA 2]HClO₃+H₂O₂→2ClO₂+H₂O+O₂↑  (Equation C)2ClO₂+H₂O₂→2HClO₂+O₂↑  (Equation D)2ClO₂+H₂O

HClO₂+HClO₃  (Equation E)2HClO₂

H₂O+Cl₂O₃  (Equation F)

In cases where chlorine dioxide is generated, it will be converted tochlorous acid by a series of reactions shown by equations C to F.

Chlorous acid (HClO₂) thus produced possesses the propensity todecompose quickly to chlorine dioxide gas or chlorine gas byinteractions among a plurality of chlorous acid molecules or by thepresence of chloride (Cl⁻) ions, hypochlorous acid (HClO), or otherreductive agents. It is, therefore, necessary to provide a long-actingchlorous acid (HClO₂) preparation effective for use as disinfectant.

Under these circumstances, it is necessary to provide a process forproducing a stable, long-acting aqueous chlorous acid (HClO₂) solution;and this is achieved by creating a transitional state to delay theprogress of the decomposition reaction through the addition of at leastone compound selected from the group consisting of inorganic and organicacids and salts or a combination thereof, to the aqueous chlorous acid(HClO₂) solution produced according to the process described in Example1 above. This process is embodied in Examples 2, 3, and 4.

Example 2

Specifically, according to Example 2, the aqueous chlorous acid (HONsolution produced according to the process described in Example 1 ismixed with inorganic acid(s) or organic salt(s), or more specifically,at least one compound selected from the group consisting of carbonatesand hydroxides or a combination thereof.

Example 3

Also, according to Example 3, the aqueous solution produced in Example 2is mixed with at least one compound selected from the group consistingof inorganic and organic acids and salts or a combination thereof.

Example 4

Moreover, according to Example 4, the aqueous solution produced inExample 1 is mixed with at least one compound selected from the groupconsisting of inorganic and organic acids and salts or a combinationthereof.

As for the above, inorganic acids can be mentioned as well as carbonicacid, phosphoric acid, boric acid, and sulfuric acid. For the inorganicsalts, carbonates and hydroxides can be mentioned as well as phosphatesand borates. More specifically, the carbonates include sodium carbonate,potassium carbonate, sodium bicarbonate, and potassium bicarbonate; thehydroxides include sodium hydroxide and potassium hydroxide; thephosphates include disodium hydrogenphosphate, sodiumdihydrogenphosphate, trisodium phosphate, tripotassium phosphate,dipotassium hydrogenphosphate, and potassium dihydrogenphosphate; andthe borates include sodium borate and potassium borate. Moreover, theabove organic acids include succinic acid, citric acid, malic acid,acetic acid, and lactic acid; and appropriate examples of the organicsalts include sodium succinate, potassium succinate, sodium citrate,potassium citrate, sodium malate, potassium malate, sodium acetate,potassium acetate, sodium lactate, potassium lactate, and calciumlactate.

In Examples 2, 3, and 4, the following transitional states aretemporarily created:Na⁺+ClO₂ ⁻⇄Na—ClO₂,K⁺+ClO₂ ⁻⇄K—ClO₂,H⁺+ClO₂ ⁻⇄H—ClO₂.These states contribute to delaying the progress of the conversion ofchlorous acid (HClO₂) to chlorine dioxide (ClO₂), which enables theformation of an aqueous chlorous acid solution that is capable ofsustaining chlorous acid (HClO₂) for an extended time and releases areduced amount of chlorine dioxide (ClO₂).

Now, the lower the pH value (the stronger the acidity) of a chlorineoxide, the stronger its bactericidal potential is known to be. Thetables below show the results of experiments on the relationship betweenpH values and bactericidal powers. In these experiments, a pathogenicEscherichia coli strain (O157:H7) was used as the test microbe. Sodiumchlorite (Wako Pure Chemical Industries, Ltd., Osaka, Japan, 80%) wasused as the test chlorine oxide. Citric acid (Wako, 98%), lactic acid(Wako, 85% to 92%), and acetic acid (Wako, 99.7%) were used as anactivator. Thirty-mL aliquots of an aqueous sodium chlorite solution(0.5 g/L, pH 9.8) were adjusted to pH 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, and8.0 by addition of citric acid, lactic acid, or acetic acid. Phenolindex measurement procedure was employed for assessment of thebactericidal effect. Ten-mL aliquots of the test solutions diluted asappropriate were transferred to test tubes, allowed to warm for at least5 minutes in a thermostatic water bath set at 20° C.±1° C. Then, 1-mLaliquots of the test microbe preparation, pre-warmed in a similarmanner, were added to the test tubes, and sampling was performed by useof a platinum loop at 2.5, 5, 10, and 15 minutes after the addition. Thesampled microbial mixtures were inoculated into a normal bouillon mediumand incubated at 37° C. for 48 hours. Bacterial growth was visuallyobserved; positive growth was designated as ‘+’ and negative growth as‘−.’

TABLE 1 Bactericidal potential for sodium chlorite solution: ActivatorCitric acid pH time (min.) 3.0 4.0 5.0 6.0 7.0 8.0 2.5 − − + + + + 5 −− + + + + 10 − − − + + + 15 − − − − + +

TABLE 2 Bactericidal potential for sodium chlorite solution: ActivatorLactic acid pH time (min.) 3.0 4.0 5.0 6.0 7.0 8.0 2.5 − − + + + + 5 −− + + + + 10 − − − + + + 15 − − − − + +

TABLE 3 Bactericidal potential for sodium chlorite solution: ActivatorAcetic acid pH time (min.) 3.0 4.0 5.0 6.0 7.0 8.0 2.5 − − + + + + 5 −− + + + + 10 − − − + + + 15 − − − − + +

As shown in the tables above, aqueous sodium chlorite solutions havingpH 7.0 or higher fell short of completely eradicating the test E. colistrain in 15 minutes. However, complete eradication was achieved in 2.5minutes when the pH was adjusted to 4.0 or lower, in 10 minutes when thepH was adjusted to 5.0, and in 15 minutes when the pH was adjusted to6.0. These findings demonstrate a higher bactericidal potential forsodium chlorite solution having a more acidic pH value. These findingsalso demonstrate that the difference in the type of activator poses nosignificant difference in the bactericidal power of sodium chloritesolution.

Thus, chlorite solution exhibits a stronger bactericidal effect when itis more acidic. However, when its pH is strongly acidic, for example, ata value in the order of 2.0, its applicable areas in food industry arelimited because of its negative effects such as denaturation of proteincomponents of the sterilized food items.[CHEMICAL FORMULA 3]5ClO₂⁻+4H⁺→4ClO₂+5Cl⁻+2H₂O(5NaClO₂+4CH₃COOH→4ClO₂+4CH₃COONa+NaCl+2H₂O)  (a)3ClO₂ ⁻→2ClO₃ ⁻+Cl⁻(3NaClO₂→2NaClO₃+NaCl) Autodecomposition  (b)ClO₂ ⁻→Cl⁻+2O  (c)

The above Chemical Formula 3 represents the decomposition of chloritecompounds in an acidic solution. When the solution has a lower (moreacidic) pH value, the decomposition of chlorite compounds is enhanced,that is, the absolute kinetic rates of the reactions represented byabove equations (a), (b), and (c) become elevated. Practically, with adecreased pH value, the dominance of the reaction products of equation(a) is reduced. However, with a decrease in pH, the total decompositionpercentage shifts to a larger value, resulting in an increased amount ofClO₂ (chlorine dioxide) generated. Therefore, the lower the pH value ofthe aqueous solution becomes, the more likely it makes the disinfectantharmful to human health and disinfection operation awkward due to therelease of toxic and irritating ClO₂ gas, although it improves thebactericidal and bleaching potentials. Also, a lower pH solution renderschlorous acid more unstable, and thereby enhances the conversion ofchlorous acid to chlorine dioxide. As a result, the duration of thebactericidal activity is reduced.

Therefore, when an aqueous chlorous acid (HClO₂) solution is mixed withany of the above-mentioned inorganic acids, inorganic salts, organicacids, or organic salts, the solution should be adjusted to pH 3.2 to7.0, from the viewpoint of suppressing chlorine dioxide release andmaintaining the bactericidal activity. The pH values should reach ashigh as allowed by the requirements on the bactericidal activityconditions within the above range. This contributes to producing anaqueous chlorous acid solution emitting a reduced amount of chlorinedioxide (ClO₂) by slowing the conversion of chlorous acid to sodiumchlorite (NaClO₂) while maintaining chlorous acid (HClO₂) for anextended time.

As described below, to confirm the effects of the present invention, thefollowing samples were prepared and subjected to measurement.

First, a chlorous acid solution prepared according to Example 1 wasmixed with 1 mol/L sodium carbonate to pH 5.7. The solution(corresponding to an aqueous chlorous acid solution prepared accordingto Example 2) was added to a 0.05 mol/L sodium borate/succinate buffer(pH 5.7) to give a 3% chlorous acid content. To sum up, this solution(corresponding to an aqueous chlorous acid solution prepared accordingto Example 3) was prepared by adding to an aqueous chlorous acidsolution an inorganic salt compound, followed by addition of acombination of an inorganic salt and an organic salt as buffer. Thissolution was termed sample A.

Second, the chlorous acid solution prepared according to Example 1 wasmixed with 1 mol/L sodium carbonate to pH 5.7. Subsequently, thissolution was mixed with deionized water to give a 3% chlorous acidcontent. In other words, this solution (corresponding to an aqueouschlorous acid solution prepared according to Example 2) was prepared byadding to an aqueous chlorous acid solution an inorganic salt compound.This solution was termed sample B.

Moreover, an aqueous solution containing 25.0% chlorous acid (Wako, 80%)was mixed with 1 mol/L solution of citric acid (Wako, 98%) to pH 2.6.The resulting solution was mixed with deionized water to give a 3%chlorous acid content. This process corresponds to a conventionaltechnique for preparing above-mentioned ASC solution. This solution wastermed sample C.

Furthermore, the chlorous acid solution prepared according to Example 1was added to 0.05 mol/L sodium borate/succinate buffer (pH 6.8) to givea final pH of 5.7 and a chlorous acid content of 3%. In other words,this solution (corresponding to an aqueous chlorous acid solutionprepared according to Example 4) was prepared by adding to an aqueouschlorous acid solution a combination of an inorganic salt and an organicsalt as buffer. This solution was termed sample D.

The time-course stability of chlorous acid (HClO₂) in each sample wascompared by measurement of UV spectra and molecular content. Themeasurement samples all contained 3% chlorous acid (HClO₂). UV spectrummeasurement was conducted on a spectrophotometer adjusted to provide anabsorbance of approximately 1 at the wavelength of maximum absorptionwhen used to measure sample solutions diluted with an appropriate volumeof ion exchanged water. Measurement of chlorous acid content wasperformed by the iodometric titration method described hereafter.Samples were aerated in an airtight container to eliminate chlorinedioxide dissolved in the samples. Then, approximately 10 g of eachsample was measured accurately, and water was added to make the volumeprecisely 100 mL. These solutions were designated as test solutions. Avolume of each of the test solutions containing approximately 0.06 g ofchlorous acid (HClO₂) was accurately measured, placed in an iodineflask, mixed with 12 mL of sulfuric acid (3→100), and water was added tomake the volume approximately 55 mL. Immediately after adding 4 g ofpotassium iodide to the solution, the flask was stoppered and kept in adark place for 15 minutes. Titration was performed by using 0.1 mol/Lsodium thiosulfate and a starch indicator, and the amount of chlorousacid in the solution was determined by the formula: 1 mL of 0.1 mol/Lsodium thiosulfate solution=0.001711 g of HClO₂. Separately, blank testswere conducted for correction. The test solutions were stored in a darkplace for preservation tests. Aliquots of the test solutions weresubjected to measurements of chlorous acid content, UV absorption, andpH values immediately after preparation and at 1, 2, 3, 24, 48, 72, 96,120, 240, 480, and 720 hours after preparation.

Consequently, the spectrometric measurement results identified twoabsorption peaks in the wavelength range of 248 to 420 nm immediatelyafter preparation of samples A, B, C, and D: one absorption peak in thevicinity of 260 nm corresponding to acidic chlorite ions (H⁺+ClO₂ ⁻) andthe other absorption peak near 350 nm corresponding to chlorine dioxide(ClO₂). These results demonstrate the presence of chlorous acid (HClO₂)(FIGS. 1, 5, 9, and 13), because they indicate the concurrently ongoingchain of reactions shown in Chemical Formula 4 involving chlorous acid(HClO₂), chlorine dioxide (ClO₂), and acidic chlorite ion (ClO₂ ⁻).[CHEMICAL FORMULA 4]Chain of reactions of chlorous acid, chlorine dioxide, and acidicchlorite ion

For sample C, although the presence of two peaks was clearlyrecognizable at 1 hour (FIG. 10), the two peaks became less visible at24 hours (FIG. 11), and thereafter the measurement results presented asingle peak near 350 nm (FIG. 12). These changes indicate the progressof conversion of chlorous acid to chlorine dioxide.

Meanwhile, samples A, B, and D showed two peaks near 260 and 350 nmafter 30 days (FIGS. 4, 8, and 16). It follows that the aqueous chlorousacid solution of the present invention provides more stable chlorousacid solutions than conventional disinfectants.

Of these, sample B, as shown in FIGS. 5, 6, 7, and 8, which illustratethe time course of changes in UV absorption, demonstrates marked changesin the shape of the two peaks as the period extends to 10, 20, and 30days. On the contrary, samples A and D maintained on day 30 the twopeaks observed on day 0 (FIGS. 1, 2, 3, 4, 13, 14, 15, and 16). Theseresults suggest little change over time for samples A and D in thecomposition of chlorous acid, chlorite ions, chlorine dioxide, and otherchlorine oxide compounds. It is apparent, therefore, that Example 2(involving addition of inorganic salt), Example 3 (involving addition ofinorganic salt followed by addition of inorganic acid and salt), andExample 4 (involving addition of organic acid and salt) preserve theinitial composition of the solution better than conventionaldisinfectants.

Table 4 depicts the changes over time in chlorous acid content. Theresults show that sample C (ASC) lost half of the initial chlorous acidcontent within 2 hours after preparation, and lost it almost completelyon day 4. On the other hand, samples A, B, and D retained much of theinitial chlorous acid content even on day 30. Therefore, the aqueouschlorous acid solutions of the present invention are superior toconventional disinfectants, because they maintain chlorous acid contentfor an extended time.

Of these, samples A and D maintained the initial chlorous acid content(prepared on day 0) for 30 days. This indicates that the aqueouschlorous acid solutions prepared according to Examples 3 and 4 possessthe highest capacities of maintaining the stability of chlorous acidover time.

TABLE 4 Comparison of maintenance of chlorous acid (HClO₂) in solutions(HClO₂ content 3%) time course 0 hr 1 hr 2 hr 3 hr 24 hr 48 hr 72 hr 96hr 120 hr 240 hr 480 hr 720 hr Sample A + + + + + + + + + + + + 3.0 3.03.0 3.0 3.0 3.0 3.0 3.0 3.0 2.9 2.8 2.7 Sample B + + + + + + + + + + + +3.2 3.2 3.2 3.2 3.1 3.0 3.0 2.9 2.9 2.6 2.3 2.1 Sample C + + − − − − − −− − − − 3.0 1.9 1.6 1.4 1.0 0.9 0.9 ND ND ND ND ND SampleD + + + + + + + + + + + + 3.0 3.0 3.0 3.0 2.9 2.9 2.9 3.0 3.0 2.9 2.92.7Lower figures in each cell represent HClO₂ content (%), and upper signsdesignate UV measurement results: +, presence of two absorption peaksidentified at 260 and 350 nm; −, presence of a maximum absorption peakidentified only in the vicinity of 350 nm.

FIG. 17 shows the time course of changes in pH values for samples A, B,C, and D. The pH value of sample B, which was initially set at 5.7,temporarily elevated to the order of 6, and gradually decreasedthereafter. On the other hand, sample A, which had an initial pH valueof 5.8, retained the pH level after 30 days, indicating theeffectiveness of the buffering action. Likewise, sample D, which had aninitial pH value of 5.7, maintained the pH level after 30 days,indicating the effectiveness of the buffering action. These resultsindicate that pH can be stabilized either by directly adding a bufferagent to the aqueous solution or by adding a buffer agent after pHadjustment with sodium carbonate.

As can be seen from the above, the aqueous solution obtained byacidifying an aqueous sodium chlorite solution, as in the process forproducing ASC, rapidly loses the chlorous acid (HClO₂) content by ahighly accelerated conversion of the acid to chlorine dioxide (ClO₂).The aqueous solutions obtained according to the present invention,however, adjust the shortage or excess of hydrogen ions resulting fromoxidation-reduction reactions of chlorine oxides while buffering the pHwithin a narrow range. Consequently, stabilization of pH contributes topreserving the transitional state of chlorous acid (HClO₂): H⁺+ClO₂⁻⇄HClO₂, and this allows for maintaining the chlorous acid content bysustaining the stoichiometric balance of molecules and ions in theaqueous chlorous acid solution.

These observations argue that the process of the present inventionexhibits superiority in providing an aqueous solution that has a highbactericidal activity and an extended stability of chlorous acid(HClO₂).

The present invention achieves a long-term stabilization of chlorousacid, which has a high bactericidal potential. It will, therefore,enable commercial distribution of aqueous chlorous acid solutions on themarket that have not been successfully circulated as sales products. Itwill contribute to widespread social adoption of chlorous acid, which isuseful as disinfectant.

So far, the present invention has been explained based on embodimentswith reference figures and tables. However, the present invention is notrestricted to these implementations, and can be practiced in variousmodes within the scope of the accompanying claims.

INDUSTRIAL APPLICABILITY

The aqueous chlorous acid solution obtained according to the presentinvention can be applied for bleaching, removal of bloodstains, andother similar uses, in addition to bactericidal purposes.

The invention claimed is:
 1. A process for producing an aqueous chlorousacid solution, comprising the steps of: forming a chlorousacid-containing solution using aqueous sodium chlorate solution andsulfuric acid or aqueous solution thereof as starting materials; andadding to the chlorous acid-containing solution at least one compoundselected from the group consisting of inorganic and organic acids andsalts or a combination thereof, to provide long-term stabilization ofsaid chlorous acid for delaying the conversion of chlorous acid tochlorine dioxide and produce the aqueous chlorous acid solution in aform that is capable of sustaining chlorous acid for an extended time of30 days, wherein the pH of the aqueous chlorous acid solution is in therange of 3.2 to 7.0.
 2. A process for producing an aqueous chlorous acidsolution, comprising the steps of: forming a chlorous acid-containingsolution using aqueous sodium chlorate solution and sulfuric acid oraqueous solution thereof as starting materials; and adding to thechlorous acid-containing solution at least one compound selected fromthe group consisting of inorganic acids and salts or a combinationthereof, in order to provide long-term stabilization of said chlorousacid for delaying the conversion of chlorous acid to chlorine dioxideand produce the aqueous chlorous acid solution in a form that is capableof sustaining chlorous acid for an extended time of 30 days, wherein thepH of the aqueous chlorous acid solution is in the range of 3.2 to 7.0.3. A process for producing an aqueous chlorous acid solution, comprisingthe steps of: forming a chlorous acid-containing solution using aqueoussodium chlorate solution and sulfuric acid or aqueous solution thereofas starting materials; adding to the chlorous acid-containing solutionat least one compound selected from the group consisting of inorganicacids and salts or a combination thereof; and further adding at leastone compound selected from the group consisting of inorganic and organicacids and salts or a combination thereof, to provide long-termstabilization of said chlorous acid for delaying the conversion ofchlorous acid to chlorine dioxide and produce the aqueous chlorous acidsolution in a form that is capable of sustaining chlorous acid for anextended time of 30 days, wherein the pH of the aqueous chlorous acidsolution is in the range of 3.2 to 7.0.
 4. The process for producing anaqueous chlorous acid solution according to claim 1, wherein saidinorganic acids include carbonic acid, phosphoric acid, boric acid, orsulfuric acid.
 5. The process for producing an aqueous chlorous acidsolution according to claim 1, wherein said inorganic salts includecarbonates, hydroxides, phosphates, or borates.
 6. The process forproducing an aqueous chlorous acid solution according to claim 5,wherein said carbonates include sodium carbonate, potassium carbonate,sodium bicarbonate, or potassium bicarbonate.
 7. The process forproducing an aqueous chlorous acid solution according to claim 5,wherein said hydroxides include sodium hydroxide or potassium hydroxide.8. The process for producing an aqueous chlorous acid solution accordingto claim 5, wherein said phosphates include disodium hydrogenphosphate,sodium dihydrogenphosphate, trisodium phosphate, tripotassium phosphate,dipotassium hydrogenphosphate, or potassium dihydrogenphosphate.
 9. Theprocess for producing an aqueous chlorous acid solution according toclaim 5, wherein said borates include sodium borate or potassium borate.10. The process for producing an aqueous chlorous acid solutionaccording to claim 1, wherein said organic acids include succinic acid,citric acid, malic acid, acetic acid, or lactic acid.
 11. The processfor producing an aqueous chlorous acid solution according to claim 1,wherein said organic salts include sodium succinate, potassiumsuccinate, sodium citrate, potassium citrate, sodium malate, potassiummalate, sodium acetate, potassium acetate, sodium lactate, potassiumlactate, or calcium lactate.
 12. The process for producing an aqueouschlorous acid solution according to claim 1, wherein in the aqueoussolution, a chain of reaction represented by

is ongoing.
 13. The process for producing an aqueous chlorous acidsolution according to claim 2, wherein in the aqueous solution, a chainof reaction represented by

is ongoing.
 14. The process for producing an aqueous chlorous acidsolution according to claim 3, wherein in the aqueous solution, a chainof reaction represented by

is ongoing.